The use of simulcast transmission to increase the effective coverage area of land-mobile radio systems is well known in the art. In simulcast transmission, two or more transmitters, broadcasting identical information simultaneously on the same frequency, are located such that contiguous coverage is available over a larger area than can be covered by the transmitters acting alone. Simulcast transmission systems require that the base band signals be transmitted at a precisely controlled time. If the signal is transmitted by the various transmitters at the wrong time, distortion occurs in the area where signals from both transmitters are received with similar signal strengths. This distortion effect is present when the various signals arrive at the receiving end with even slight phase, or timing, differences with respect to each other.
FIG. 1 shows a simplified graphical representation of a typical simulcast radio transmission system 100. The system comprises two base stations, or remote site transmitters 101, 103. Remote site transmitter 101 has an accompanying coverage area 111, within which subscriber units 105, 107 are able to receive transmitted messages via transmissions 119, and 117, respectively. Similarly, remote transmitter site 103 has an accompanying coverage area 113, within which subscriber units 107, 109 are able to receive transmitted messages via transmissions 118, and 121, respectively. Note that coverage areas 111 and 113 have, by design, an overlapping coverage area 115, within which a subscriber unit 107 receives transmissions from both transmitters. It is this overlapping coverage area 115 that incorporates simulcast technology in order to enhance the respective coverage areas of the transmitter sites involved. Accordingly, these transmissions 117, 118 are perceived by subscriber unit 107 as a single signal.
FIG. 2 shows a simplified block diagram of a typical simulcast radio transmission system 200. A typical transmission sequence begins when a message source 202 (e.g., console, radio, key management center (KMC), etc.) sends a message signal to be transmitted to one or more coverage areas. Upon receipt of the message signal, the controller 204 distributes the message signal to one or more remote site transmitters (e.g. 208, 210). This distribution is typically done through an expensive microwave distribution system 206 which comprises, for example, interconnect links 214, 216. When the message signal has been received at the remote site transmitter, for example transmitter 208, the message is transmitted to an awaiting mobile subscriber unit within the coverage area for that site.
Because of the critical timing requirements of simulcast transmission, the interconnect links must be precisely calibrated, or netted, using a variable delay within the receiving modem at each remote site transmitter. Such calibration maintenance is required to insure that the total propagation delays are identical across all interconnect links. Microwave distribution is used because other methods, such as telephone lines, do not maintain constant propagation delays over time, thus demanding more frequent maintenance cycles. Additionally, custom-built modems are required in order to maintain the required frequency response tracking and provide needed features. These are very costly when compared with off-the-shelf modems that are readily available. Furthermore, the system must be re-netted occasionally, which is usually performed from a central site at which signals from the transmitter site can be received and the signal delay measured. Test signals and expensive measurement equipment are used to determine signal delays for each site, while a data network is used to adjust these measured delays. Simpler, less expensive calibration systems do exist, but these do not generally perform well on systems using high speed data rates, such as those in a typical simulcast environment operating at or beyond 9600 bits/second.
Recent simulcast systems utilize highly accurate time sources, such as those available using Global Positioning System timing receivers, to synchronize the transmission time at the various simulcast transmitter sites. These systems synchronize digital signals and voice band signals that have been distributed to the transmitter sites through digital networks by appending "Launch Time" information to the signal to tell the transmitter the time at which to transmit the signal. In systems, selective call tones are generated and added by the transmitter since low frequency tones typically will not pass through a voice band network. Selective call tones, such as private line (PL) codes, are used in communication systems for unit identification purposes. If used, these tones are continuously transmitted, along with other information, and provide for a receiver to receive the information upon detecting a desired tone. These tones are highly phase sensitive and must be transmitted with the same phase from the remote site transmitters in order to prevent falsing at the receiving units. A problem arises in how to control the phase of the selective call tone especially when interruptions of the distribution network can occur and are required to have minimum impact. In the current state of the art, an interruption in information transfer to one of the plurality of simulcast transmitters leads to that transmitter's selective call tone being permanently out of phase with the selective call tone of the other transmitters.
Accordingly, there exists a need for a simulcast radio communication system which is capable of meeting the critical timing and phase requirements of simulcast transmission.